4. Environmental Setting, Impacts, and Mitigation Measures Geology and Soils 4.5 Geology and Soils This section presents geologic, soils and seismic conditions in the Inner Harbor Specific Plan Area and evaluates the potential for the development under the Specific Plan to result in significant impacts related to exposing people or structures to unfavorable geologic hazards, soils, and/or seismic conditions. A review of the applicable regulatory framework for the Specific Plan implementation is also provided. Potential impacts are discussed and evaluated, and appropriate mitigation measures are identified where necessary. A number of site-specific reports for sites in proximity to and within the Specific Plan Area were used to compile the description of existing conditions in the Plan Area. This section also presents a project-level analysis of the Harbor View project, which is based on a site-specific reports prepared for that project site. CEQA requires the analysis of potential adverse effects of a project on the environment. While potential effects of the environment on the project are arguably not required to be analyzed or mitigated under CEQA, this section nevertheless analyzes potential effects of the geology and soils on the Specific Plan implementation as set forth in CEQA Guidelines, Appendix G, Significance Criteria, and in order to provide information to the public and decision-makers. As such, the potential adverse effect of existing risk levels for expansive soils or landslides on proposed project residents is analyzed below. 4.5.1 Environmental Setting Regional Setting The Plan Area lies within the geologically complex region of California referred to as the Coast Ranges geomorphic province.1 The Coast Ranges province lies between the Pacific Ocean and the Great Valley (Sacramento and San Joaquin valleys) provinces and stretches from the Oregon border to the Santa Ynez Mountains near Santa Barbara. Much of the Coast Range province is composed of marine sedimentary deposits and volcanic rocks that form northwest trending mountain ridges and valleys, running subparallel to the San Andreas Fault Zone. The relatively thick marine sediments dip east beneath the alluvium of the Great Valley. The Coast Ranges can be further divided into the northern and southern ranges, which are separated by the San Francisco Bay. The San Francisco Bay lies within a broad depression created from an east-west expansion between the San Andreas and the Hayward fault systems. West of the San Andreas Fault lies the Salinian Block, a granitic core that extends from the southern end of the province to north of the Farallon Islands. The Northern Coast Ranges are comprised largely of the Franciscan Complex or Assemblage, which consists primarily of graywacke, shale, greenstone (altered volcanic rocks), basalt, chert (ancient silica-rich ocean deposits), and sandstone that originated as ancient sea floor sediments. 1 A geomorphic province is an area that possesses similar bedrock, structure, history, and age. California has 11 geomorphic provinces. Inner Harbor Specific Plan 4.5-1 October 2015 Draft Environmental Impact Report D130467 4. Environmental Setting, Impacts, and Mitigation Measures Geology and Soils Franciscan rocks are overlain by volcanic cones and flows of the Quien Sabe, Sonoma and Clear Lake volcanic fields (CGS, 2002a). Project Setting Geology and Soils Review of geotechnical reports previously prepared for the Plan Area and nearby development shows the soils to consist of alluvial deposits overlain by a soft to very stiff marine clay deposit, locally known as “Bay Mud”. The reports record the depths of these soils from approximately 5 to 11 feet with groundwater found from approximately 3 to 12 feet. Bay Mud is considered to be a highly compressible material resulting in differential settlements. This differential settlement impacts proposed structure and infrastructure design as well as construction practices. Structural loads are generally too great for the Bay Mud to support with conventional foundations. Instead, support systems comprised of driven piles is commonly recommended. Settlement across this compressible material is dependent upon the thickness of Bay Mud in conjunction with the thickness of fill at any given location. To minimize settlement in areas receiving fill a surcharge program is often utilized. Surcharging consists of placing excess fill in areas where settlement is a concern and leaving it in place a sufficient amount of time to partially pre-consolidate the material. Gravity utilities constructed in areas of Bay Mud are designed to avoid grade reversal, joint separation, and leakage. Flexible connections are used when utilities enter a building or other structure. Construction activities in areas of Bay Mud usually require the use of light grading equipment. Corrosivity tests show the Bay Mud to be moderately corrosive to concrete and metals. Corrosivity will impact the types of underground materials and connections specified for infrastructure systems (this concern is addressed by the City’s Building Code). (Fusco, 2014) Faults and Seismicity The Specific Plan Area lies within a region of California that contains many active and potentially active faults and is considered an area of high seismic activity, as shown in Figure 4.5-1and described in Table 4.5-1.2 The U.S. Geological Survey (USGS) along with the California Geological Survey and the Southern California Earthquake Center formed the 2 An “active” fault is defined by the State of California as a fault that has had surface displacement within Holocene time (approximately the last 11,000 years). A “potentially active” fault is defined as a fault that has shown evidence of surface displacement during the Quaternary (last 1.6 million years), unless direct geologic evidence demonstrates inactivity for all of the Holocene or longer. This definition does not, of course, mean that faults lacking evidence of surface displacement are necessarily inactive. “Sufficiently active” is also used to describe a fault if there is some evidence that Holocene displacement occurred on one or more of its segments or branches (Hart, 20072007). Inner Harbor Specific Plan 4.5-2 October 2015 Draft Environmental Impact Report D130467 Active Fault Potentially Active LEGEND C layton Clayton fault fault ault ho f Moc lt au s f Pleasanton fault m Cal Pl lia averas fault eas il anton f W ault Ca lavera s fault ault Calaveras fault Mission f ward fault n fault Hay Hayward fault Missio Hayward fault Hayward fault ult Crosley fa Silver Creek fault Figure 4.5-1 SITE PROJECT San Jose fault Regional Fault Map lt Inner Harbor Specific Plan . 130467 fau ss nford Sta lt fau ta Vis San Andre te as fault Sa on n And M reas f ault lt San Andreas fault anada fau San Andreas fault C t Pilarc itos faul aul t f s e y e R t n i Seal Cove fault Po 5 Miles 0 SOURCE: Jennings, 2010 4. Environmental Setting, Impacts, and Mitigation Measures Geology and Soils TABLE 4.5-1 ACTIVE FAULTS IN THE PROJECT SITE VICINITY Maximum Moment Distance and Magnitude Direction from Recency of Fault Historical Earthquake Fault Project Movement Classificationa Seismicityb (Mw)c San Andreas 4 miles Historic (1906; Active M 7.1, 1989 7.9 southwest 1989 ruptures) M 8.25, 1906 M 7.0, 1838 Many <M 6 Hayward 15 miles Historic (1868 Active M 6.8, 1868 7.1 northeast rupture) Many <M 4.5 San Gregorio 20 miles Prehistoric Active n/a 7.3 southwest (Sometime prior to 1775 but after 1270 A.D.) Calaveras 22 miles east Historic (1861 Active M 5.6–M 6.4,1861 6.8 1911, 1984) M 6.2, 1911, 1984 Concord–Green 30 miles Historic (1955) Active Historic active creep 6.7 Valley northeast Marsh Creek– 32 miles Historic (1980 Active M 5.6 1980 6.9 Greenville northeast rupture) Rodgers Creek 40 miles north Historic Active M 6.7, 1898 7.0 M 5.6, 5.7, 1969 a See footnote 2 b Richter magnitude (M) and year for recent and/or large events. The Richter magnitude scale reflects the maximum amplitude of a particular type of seismic wave. c Moment Magnitude (Mw) is related to the physical size of a fault rupture and movement across a fault. Moment magnitude provides a physically meaningful measure of the size of a faulting event (CGS, 2002b). The Maximum Moment Magnitude Earthquake, derived from the joint CGS/USGS Probabilistic Seismic Hazard Assessment for the State of California, 1996. (Peterson, 1996). SOURCES: Hart, 20072007; Jennings, 20102010; Peterson, 1996; USGS, 2003. Working Group on California Earthquake Probabilities which has evaluated the probability of one or more earthquakes of magnitude 6.7 or higher occurring in the state of California over the next 30 years beginning in 2014. The result of the most recent evaluation known as the Uniform California Earthquake Rupture Forecast (UCERF3) indicated a 72 percent likelihood that such an earthquake event will occur in the Bay Area region (USGS, 2015). Richter magnitude is a measure of the size of an earthquake as recorded by a seismograph, a standard instrument that records groundshaking at the location of the instrument. The reported Richter magnitude for an earthquake represents the highest amplitude measured by the seismograph at a distance of 100 kilometers from the epicenter. Richter magnitudes vary logarithmically with each whole number step representing a tenfold increase in the amplitude of the recorded seismic waves. Earthquake magnitudes are also measured by their Moment Magnitude (Mw) which is related to the physical characteristics of a fault including the rigidity of the rock, the size of fault rupture, and movement or displacement across a fault (CGS, 2002b). Inner Harbor Specific Plan 4.5-4 October 2015 Draft Environmental Impact Report D130467 4. Environmental Setting, Impacts, and Mitigation Measures Geology and Soils Ground movement during an earthquake can vary depending on the overall magnitude, distance to the fault, focus of earthquake energy, and type of geologic material.